How far off are we technology wise to have a tri engined aircraft the size of the A380 or is this something that simply just wont be considered?

The demise of the tri engines is surprising to me when we have some really large and powerful engine options these days, surely three engines on a massive airliner as the A380 or similar would be good economics and also please the green brigade.

Wouldnt it also be great for all of us entrhusiasts to have a new triple engined good looker in the skies again.

There isnt really a dark side to the moon, as a matter of fact its all dark!

I also dearly miss the Trijets, as they were my all time favorite a/c (especially the L1011 ) However i doubt they will be coming back....why make a Trijet when you can make a same sized aircraft, that is just as effecient, but has less engines to maintain?

Quoting NEMA (Thread starter):How far off are we technology wise to have a tri engined aircraft the size of the A380 or is this something that simply just wont be considered?

One can understand the aesthetic argument for a new trijet but I would imagine that the increased structural weight needed at the back end would make it uneconomical in spite of increased maintenance cost.

I certainly wish someone would design a big new trijet, but there are many costs associated with the design and mainenance of a trijet that are likely to offset whatever operational efficiency, if any, you'd gain by having one less engine.

- Designing an intake for engine #2 may be prohibitively expensive, especially considering the size of the engine that will need to live in the back (GE-90 115 sized).

-Mounting an engine in the tail, especially a great big one, and especially at the base of the H-stab (like a DC-10) requires more structural weight more wing-mounting. Additionally, the weight of 2 engines hung from each wing provides some bending relief in flight, which permits a lighter wing.

-Maintaining an engine either in the tail or way up by the H-stab is more time consuming, and therefore, more expensive than maintaining a wing-mounted engine.

Dassault Mercure: the plane that has Boeing and Airbus shaking in their boots.

Quoting NEMA (Thread starter):The demise of the tri engines is surprising to me when we have some really large and powerful engine options these days, surely three engines on a massive airliner as the A380 or similar would be good economics and also please the green brigade.

Quoting NEMA (Reply 2):But this is exactly my point, Why make a four engined jet if a tri jet can do the business with todays engine power?

They're not practical. The advantages of deleting an engine are more than negated by the difficulties involved in putting an engine on the centerline. Structural weight increase, intake issues, maintenance issues...

"There are no stupid questions, but there are a lot of inquisitive idiots."

Quoting NEMA (Thread starter):How far off are we technology wise to have a tri engined aircraft the size of the A380 or is this something that simply just wont be considered?

Technology-wise it's not a problem. You could build an A380-sized trijet today with three 105,000 lbs thrust engines.

Quoting NEMA (Thread starter):The demise of the tri engines is surprising to me when we have some really large and powerful engine options these days, surely three engines on a massive airliner as the A380 or similar would be good economics and also please the green brigade.

The economics don't work for a tube-wing configuration because of the mounting/weight/maintenance challenges with the middle engine. If we ever go to BWB's, trijets make a lot more sense.

I'd make that plane in the picture out of CFRP, update the cockpit, delete the No. 2 engine, hang one GEnx Mk2 under each new, redesigned wing, and perhaps use taller landing gears if needed - then call it a day.

Quoting Tdscanuck (Reply 7):The economics don't work for a tube-wing configuration because of the mounting/weight/maintenance challenges with the middle engine. If we ever go to BWB's, trijets make a lot more sense.

This is what I don't get. How could it work for BWB when it's essentially a threefold increase of the same challenges?

Quoting DEVILFISH (Reply 8):I'd make that plane in the picture out of CFRP, update the cockpit, delete the No. 2 engine, hang one GEnx Mk2 under each new, redesigned wing, and perhaps use taller landing gears if needed - then call it a day.

Hehe. Well I don't know if you would manage the thrust needed. By my back of the envelope math you would need 180k+ of thrust per engine to fly an aircraft of the 747s weight on two engines.

Incidentally, I find it interesting that Boeing went with an S-duct in their proposal instead of a fin mount.

"There are no stupid questions, but there are a lot of inquisitive idiots."

Quoting Starlionblue (Reply 9):By my back of the envelope math you would need 180k of thrust per engine to fly an aircraft of the 747s weight on two engines.

I'm wondering, since the B743 was 18'-10" shorter than the 748 and carried 101 less passengers in a 3-class layout - if all-composite construction would drop the weight enough to permit two 150Klbf engines excess thrust to pass the one-engine-out requirement?

Quoting Starlionblue (Reply 9):Incidentally, I find it interesting that Boeing went with an S-duct in their proposal instead of a fin mount.

Didn't the design predate their acquisition of MDD that adopting the "banjo" configuration could have led to patent infringement? Which leads to the question "Did Lockheed license the design to them"?

I would like to see this happen as much as anyone, but unfortunately we'll all have to dream on. For widebodies, all we have now are Boeing and Airbus; two companies who have never specialized in the tri-jet design for a long range aircraft. The chances of a VLA tri-jet went out the window when McDonnell Douglas was taken over, who's engineers knew as much as anyone when it came to the design.

I have no doubt in my mind that McDonnell Douglas or Lockheed could produce such an aircraft if the funding was available. The tail heavy weight problem that is argued could be solved with an extension of the fuselage forward of the wing. The MD-XX Stretch would have been quite an aircraft, too bad.

The next trijet probably will not be in the current configuration as many have said, it will be some form of lifting body-BWB. It always gets to me when people say something won't happen because of x reason; all of engineering is a compromise. If the DC-3 had been a flying wing, then everything today would have been a flying wing. Then somebody would come up with this "tube with wings" concept and we'd all freak out because its not nearly as aerodynamically perfect as the flying wing.

There's no law against three engines. I guarentee you a threeholer could be made just as efficient as the 747, maybe even the A380, its just waiting for someone to break the mold and try something outside the box. Remember, aerodynamics aren't going to change. Materials are constantly getting stronger and easier to form into complex compound curves. Every day we're getting closer to actually being able to make that perfect shape. And theres a good chance that shape will have three engines on it.

Quoting DEVILFISH (Reply 10):I thought it was the SP photoshopped with three engines?

Nono. Authentic Boeing brainstorm.

Quoting DEVILFISH (Reply 10):I'm wondering, since the B743 was 18'-10" shorter than the 748 and carried 101 less passengers in a 3-class layout - if all-composite construction would drop the weight enough to permit two 150Klbf engines excess thrust to pass the one-engine-out requirement?

Probably not. As I understand it, 748 will have 4x60k or so. This is still equivalent to 2x180.

Quoting DEVILFISH (Reply 10):Didn't the design predate their acquisition of MDD that adopting the "banjo" configuration could have led to patent infringement? Which leads to the question "Did Lockheed license the design to them"?

Boeing had experience of an S-duct with the 727. But I think they just went for one and that was that. Fin mounting has plenty of issues.

"There are no stupid questions, but there are a lot of inquisitive idiots."

Eh? The conventional construction 773 already carries as many passengers (368) using two RR Trent 892 with only 90,000 lbf thrust, or 93,700 lbf thrust GE90-94B. The 773ER with 365 pax manages with twin 115,300 lbf thrust GE90-115B. What more if those were CFRP? So, I think even 150K per side would be overkill for a totally composite, shorter 743.

Quoting Starlionblue (Reply 13):As I understand it, 748 will have 4x60k or so. This is still equivalent to 2x180.

OTOH, the four GEnx-2B67s will provide 266,000 lbf total thrust for the 250' long, still mostly metal, 970,000 lb MTOW 747-8 Intercontinental. Make it an all-composite, radically lighter airframe and two 150K engines might just be in the ballpark.

Quoting DEVILFISH (Reply 8):Quoting Tdscanuck (Reply 7):
The economics don't work for a tube-wing configuration because of the mounting/weight/maintenance challenges with the middle engine. If we ever go to BWB's, trijets make a lot more sense.

This is what I don't get. How could it work for BWB when it's essentially a threefold increase of the same challenges?

Because it's not the same challenges. The engines on a BWB, even if "rear" mounted, are far closer to the CG than they are on a tube-and-wing. You also don't have a vertical stabilizer on the centerline in the exact spot where you'd want to put the third engine.

On a BWB, all of the engine mounts are pretty much the same so the difference between two, three, or four (from a mounting point of view) is a roughly linear scale. That's not true for a tube-and-wing configuration because your centerline engine has to have a radically different mounting than your L and R engines. You get a step-change in integration complexity with an odd number of engines on a tube-and-wing airplane...that doesn't happen on a BWB.

Somewhat off-topic, but current trends do not seem to substantiate the theory that composite airframes are radically lighter. Neither the 787 or A350 represent 'radical' changes in structural efficiency... they may be incrementally better, but 'radical' seems like too strong a term... for example, compare the numbers for a 763ER and a 788.

Guys, thank you all very much for bearing with me - please keep in mind that I'm not questioning anybody's knowledge. Being inquisitive is just my way of trying to understand why things are a certain way. And learning sure doesn't come as easily to a 51 year old dog.

I totally forgot the 727. Going back to the 743, I was expecting the excess thrust to take care of its greater girth and cargo load, and whatever residual weight a CFRP version would still have over the 773ER.

Quoting Tdscanuck (Reply 15):The engines on a BWB, even if "rear" mounted, are far closer to the CG than they are on a tube-and-wing.

See, I had always assumed that the aft fuselage was a dead weight distributed along the simply-supported span between the wings and the tailplanes. And that the tailplanes carried the No. 2 engine and vert stab. I'd never considered the middle engine (or 1 & 3 for that matter) inducing tremendous moments on the aft fuselage structure - I thought of it mostly as axial thrust.

Quoting Tdscanuck (Reply 15):You also don't have a vertical stabilizer on the centerline in the exact spot where you'd want to put the third engine.

Couldn't this be solved by twin tailfins, or would those just add weight and complexity? Which in the final analysis would confirm that the benefits of having one less engine burning ever expensive fuel are far outweighed by the simplicity, ease of maintenance and structural efficiency of quads - and even more powerful and heavier twin engines?

Quoting WingedMigrator (Reply 16):Neither the 787 or A350 represent 'radical' changes in structural efficiency... they may be incrementally better, but 'radical' seems like too strong a term...

I guess I'm too sold on the CFRP 787 being "revolutionary" - thus falling prey to the common, "big" misconception that it's "radically" lighter.

Quoting DEVILFISH (Reply 14):Quoting Starlionblue (Reply 13):
As I understand it, 748 will have 4x60k or so. This is still equivalent to 2x180.

OTOH, the four GEnx-2B67s will provide 266,000 lbf total thrust for the 250' long, still mostly metal, 970,000 lb MTOW 747-8 Intercontinental. Make it an all-composite, radically lighter airframe and two 150K engines might just be in the ballpark.

266k on a quad means 199.5k (66.5 x3) with an engine out. The airframe would thus have to be at least 25% lighter to enable a twin with 150k x2. And the wings would have to be stronger since there are no outboards to provide bending relief. And the rudder/fin would have to be bigger (heavier) for engine out authority. 25%+ is a huge number in this context. I still don't see it.

"There are no stupid questions, but there are a lot of inquisitive idiots."

If bending relief is that big of a problem than you can always mount the engines farther outboard. The problem is not that a twin or tri 747/A380 sized airplane can't be made, its that it would be a major deviation from the current trend. Think of it like cars, obviously there are three wheeled cars out there, but 99% of passenger vehicles are four wheels, tractor trailers have 18, etc. Its not so much about engineering as it is about economics. If you can use the same basic starting block you've been using for years then why would you spend millions or billions to literally start from scratch.

(Un?)Fortunately for aviation, airplanes are designed much more for absolute performance; whereas a great deal of car designing is geared towards cosmetics and luxury. So theres a better chance of seeing radical new designs in aviation because eventually we'll max out the efficiency of tubes and wings, and be forced to go to new, exciting shapes. Taking a GE90 and throwing on the tail of a 747 might not work, but 3 GE90's on a flying wing could work beautifully.

Quoting DEVILFISH (Reply 17):See, I had always assumed that the aft fuselage was a dead weight distributed along the simply-supported span between the wings and the tailplanes. And that the tailplanes carried the No. 2 engine and vert stab. I'd never considered the middle engine (or 1 & 3 for that matter) inducing tremendous moments on the aft fuselage structure - I thought of it mostly as axial thrust.

The tailplane (on virtually all aircraft) is pushing down. It's actually an additional "weight" on the fuselage. Having the weight of an engine back there, plus the weight of the supporting structure, just makes it worse.

Quoting DEVILFISH (Reply 17):Quoting Tdscanuck (Reply 15):
You also don't have a vertical stabilizer on the centerline in the exact spot where you'd want to put the third engine.

Couldn't this be solved by twin tailfins, or would those just add weight and complexity?

Quoting DEVILFISH (Reply 22):Eureka!!! Heavens be praised praise - the root of all my stupid assertions. I had assumed all along that tailplanes were providing lift to keep the tail up and level! Thank you very much for the lesson.

Indeed. You can think of the aircraft as a seesaw. The center of lift is the fulcrum, and is located between the center of gravity (in front) and the tailplane (in back). Just like a seesaw, the arrangement can be balanced and pitch can be altered by changing the downward force on one of the weights, in this case the tailplane.

Relaxed stability aircraft are a bit different, but that's another story.

"There are no stupid questions, but there are a lot of inquisitive idiots."